Technique for the deposition of gallium phosphide resistive films by cathodic sputtering

ABSTRACT

Gallium phosphide thin films manifesting resistivities within the rage of 10 to 106 ohm-centimeters are obtained by cathodic sputtering of gallium phosphide at controlled temperatures and deposition rates. The resistivity of the sputtered films may increased to 108 ohm-centimeters by annealing at elevated temperatures.

United States Patent Inventor Appl. No. Filed Patented Assignee TECHNIQUE FOR THE DEPOSITION OF GALLIUM I PHOSPIIIDE RESISTIVE FILMS BY CATI-IODIC SPU'ITERING [56] References Cited UNITED STATES PATENTS 3,39 l ,071 8/l968 Theverer 204/192 3,418,229 12/1968 Lakshmanan et aI 204/192 3,522, l 64 8/1970 Sumner 204/192 Primary Examiner.|0hn H. Mack Assistant ExaminerSidney S. Kanter Attorneys-R. J. Guenther and Edwin B. Cave ABSTRACT: Gallium phosphide thin films manifesting resistivities within the rage of 10 to 10' ohm-centimeters are obsclaimssnnwing Figs tained by cathodic sputtering of gallium phosphide at con- U.S.CI 204/192 trolled temperatures and deposition rates. The resistivity of Int. Cl C231: 15/00 the sputtered films may increased to 10" ohm-centimeters by Fleld of Search 204/ l 92 annealing at elevated temperatures.

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L: O l l 1 1 1 1 1 l I SUBSTRATE TEMPERATURE, DEG. C.

PATENTED 8EP2I 1911 3 50! 599 FIG. 3

SUBSTRATE TEMPERATURE, DEG c.

lNl/ENTOR y J. SOSN/AK PATENTEB SEPZI ml POLYCRYSTALLINE Gap \2 AMORPHOUS FILM TECHNIQUE FOR THE DEPOSITION OF GALLIUM PHOSPHIDE RESISTIVE FILMS BY CATHODIC SPUT'IERING BACKGROUND OF THE INVENTION 1 Field of the Invention This invention relates to a technique for the deposition of thin films of gallium phosphide. More particularly, the present invention relates to a technique for cathodically sputtering films of gallium phosphide.

2. Description of the Prior Art Although the sputtering phenomenon has been known for more than one hundred years, the popularity of vacuum evaporation techniques had, until recently, restricted its interest primarily to investigations directed to the interaction between the target and ions bombarding it and to the related general phenomenon of gaseous discharge.

However, in the past decade, there has been a rebirth of interest in the use of sputtering as a method for depositing thin films of controlled properties. The momentum generated by this interest has in turn lead to extensive investigations of materials suitable for processing in this manner. Among the more promising materials from a device standpoint is gallium phosphide, a composition manifesting bulk characteristics that suggest its utility in resistive and electroluminescent applications. Unfortunately, cathodic sputtering of compounds has never been seriously entertained by workers in the art due to the difficulties encountered in obtaining stoichiometric films, such difficulties being attributed to the need for ensuring difierent arrival rates at the substrate of the elements of the compounds being sputtered.

SUMMARY OF THE INVENTION In accordance with the present invention, the prior art difficulties have been successfully obviated by cathodic sputtering of gallium phosphide at controlled deposition rates and substrate temperatures. Briefly, the inventive technique involves sputtering a polycrystalline gallium phosphide sample by cathodic sputtering techniques-upon a heated substrate member at a deposition rate within a specified range and subsequently annealing the deposited stoichiometric film. Studies of the deposited films have revealed that they evidence resistivities within the range of to 10 ohm-centimeters, which may be increased to about 10 ohm-centimeters by annealing, so suggesting their use in diverse resistor applications. The described technique is also considered suitable in the sputtering of bulk doped gallium phosphide for the preparation of resistor material having a resistivity of the order of 10,000 micro-ohm cm., such as is required in thin film circuitry.

BRIEF DESCRIPTION OF THE DRAWING FIG. 3 is a graphical representation on coordinates of sub- 6 strate heater temperature in degrees Centigrade against film deposition rate in angstroms per minute showing the conditions required to obtain polycrystalline gallium phosphide in accordance with the present invention.

DETAILED DESCRIPTION With reference now more particularly to FIG. 1, there is shown a typical apparatus suitable for depositing a gallium phosphide film by cathodic sputtering. Shown in the Figure is a vacuum chamber 11 in which are disposed cathode 12 and anode l3. Cathode 12 is composed of a polycrystalline sample of gallium phosphide. A source of electrical potential 14 is shown connected between cathode l2 and anode 13. Platform 15 is employed as a positioning support for substrate 16 upon which the sputtered film is to be deposited.

The present invention may conveniently be described in more detail by reference to an illustravive example in which gallium phosphide is chosen as the cathode for use inn an apparatus similar to that shown in FIG. I.

Substrate materials suitable for use in the practice of the present invention are required to conform to the restrictions imposed by the various process steps. It is preferred that the substrate material be possessed of a smooth surface which is completely free from sharp changes in contour and able to withstand the temperatures to which it will be subjected during the course of the processing. All types of refractory materials such as glass, ceramics and high melting metals meet this requirement.

The gallium phosphide selected for use as the source material or cathode in the sputtering process may be a polycrystalline grown material evidencing a purity of the order of 99.99 percent, such material typically being obtained from commercial sources. It will be appreciated that the presence of certain dopants, such as zinc, may be desirable from the standpoint of device applications and when so desired may be present in an amount ranging up to 2 10 atoms per cm.

The substrate member is first vigorously cleaned. Conventional cleansing agents are suitable for this purpose, the choice of a specific agent being dependent upon the composition of the substrate itself. Thus, where the substrate chosen for use in the practice of the invention is glass, boiling in organic solvents is a convenient method for cleaning the surface.

The cleansed substrate is next placed in a cathodic sputtering apparatus of the type shown in FIG. 1 containing a gallium phosphide cathode with a vacuum chamber evacuated to a pressure of 10 torr. After attaining the desired background pressure, a suitable inert sputtering gas is admitted to the chamber in an amount sufficient to result in a pressure ranging up to 30 microns. The extent of the vacuum is dependent upon consideration of several factors as follows.

Increasing the inert gas pressure and thereby reducing the vacuum within the chamber increases the rate at which the gallium phosphide being sputtered is removed from the cathode and accordingly increases the rate of deposition. The maximum pressure is usually dictated by power supply limitations, since increasing the pressure also increases the current fiow between cathode l3 and anode 12 of the apparatus shown in FIG. 1. A practical upper limit in this respect is 30 microns for a sputtering voltage of 3000 volts, although it may be varied depending upon the size of the cathode, sputtering rate, etc. The ultimate maximum pressure is that which the sputtering can be reasonably controlled within the prescribed tolerances. It follows from the above discussion that the minimum pressure is determined by the lowest deposition rate which can be economically tolerated.

After the requisite pressure is attained, the substrate member is heated to a temperature within the range of 25 to 650 C., by a conventional heater. An extensive evaluation of the various factors involved in sputtering of gallium phosphide in accordance with the invention have revealed that there is a direct correlation between the substrate temperature, deposi- 5 tion rate and the type of films deposited, that is, amorphous or polycrystalline films. Thus, as described in more detail below, it has been found that in order to obtain polycrystalline gallium phosphide at temperatures of the order of 650 C., the sputtering rate must be at least angstroms per minute, whereas sputtering in a system wherein the substrate member is maintained at a temperature of the order of 25 C. requires a sputtering rate of at least 380 angstroms per minute to yield polycrystalline gallium phosphide films.

Following, the cathode member is made electrically negative with respect to the anode member. The minimum voltage necessary to produce sputtering is approximately 1000 volts. Increasing the potential difference between the cathode and the anode has the same effect as increasing the pressure, that of increasing both the rate of deposition and the current flow. Accordingly, the maximum voltage, 5000 volts, is dictated by 5 consideration of the factors which control the maximum pressure.

The balancing of the various factors of voltage pressure and the relative positions of the cathode, anode and substrate to obtain a high quality deposit is well known in the sputtering art. However, in the instant case, the sputtering rate and the temperature to which the substrate member is heated during the course of the processing is considered critical. The balancing of these factors must be such as to result in a sputtering rate ranging from approximately 80 angstroms per minute to 380 angstroms per minute over the noted temperature range of from 25 to 650 C., the lower rates corresponding with the higher temperature and vice versa.

With reference now more particularly to H0. 2, there is shown a bar graph on coordinates of substrate heater temperature in degrees centigrade against deposition rate in angstroms per minute showing the relationship between these parameters and the type of gallium phosphide film produced. It will be noted by reference to the graph that at a temperature of C., amorphous films of gallium phosphide are produced at sputtering rates ranging up to approximately 260 angstroms per minute. In order to obtain the desired polycrystalline films at this temperature, it is seen that a sputtering rate of the order of 380 angstroms per minute is required. Similarly, it will be noted that at substrate temperatures as high as 290 C., a sput- 3o tering rate of the order of 365 angstroms per minute is still required in order to obtain a polycrystalline film. However, at substrate temperatures of 420 C., this requirement changes and it is found that a sputtering rate of only 200 angstroms per minute will yield the polycrystalline gallium phosphide film. As we proceed to higher temperatures, it is found that the sputtering rate required to yield a gallium phosphide film decreases even further so that at temperatures ranging from 600 to 650 C., the required sputtering rate dips below 100 angstroms per minute. In order to determine the specific required sputtering rate at any substrate temperature within the noted range, a simple graphical representation may be plotted based on the value set forth in H6. 2. The resultant graph is shown in FIG. 3.

Accordingly, by employing a proper voltage, pressure, spacing of elements, substrate temperature and deposition rate, a polycrystalline film of gallium phosphide may be deposited upon the substrate member. Sputtering is conducted for a period of time calculated to produce the desired thickness. 0 The specific sputtering technique chosen for use in the practice of the present invention may be selected from among conventional DC sputtering radio frequency (RF) sputtering by or combined RF-DC sputtering.

For the purposes of the present invention, the minimum thickness of the layer deposited upon the substrate is approximately 500 angstroms. There is no maximum limit on this thickness, although little advantage is gained by an increase above 80,000 angstroms.

Following the deposition technique, the gallium phosphide film is annealed at a temperature within the range of 600 to 700 C. for a time period ranging from 2 to 50 hours for the purpose of removing defects in the film so deposited. The duration of heating is dictated by considerations relating to film thickness, the shorter time periods corresponding with the smaller thicknesses and the converse.

An example of the present invention is described in detail below. The example and the illustration described above are merely included to aid in the understanding of the invention and variations may be made by one skilled in the art without departing from the spirit and sco e of the invention.

EXAMBLE A sputtering apparatus similar to that shown in FIG. 1 was used to produce the gallium phosphide layer. The cathode consisted of a wafer of gallium phosphide of 99.99 percent purity which had been diced and ground flat for use therein. The gallium phosphide wafer was mounted by means of a conductive epoxy adhesive on a cooling block that forms a part of the cathode member. in the apparatus actually employed, the anode was grounded, the potential difference being obtained by making the cathode negative with respect to ground. A glass microscope slide, approximately 1% inch 3 inch in length was selected as a substrate member. The slide was first washed in a detergent such as Alconox to remove large particles of dirt and grease. Next, there followed a tap water rinse, a lO-minute boil in a 10 percent hydrogen peroxide solution, a distilled water rinse and a l0minute boil in distilled water. The cleaned slide was next inserted in the apparatus and the vacuum chamber evacuated by means of a suitable pump to a pressure of approximately 10 6 torr. after a time period within the range of 30 to 45 minutes. Next, the substrate heater was actuated and the substrate heated to a temperature of 420 C. After attaining such temperature, argon was admitted into the chamber in an amount equal to approximately 30 microns.

A direct current voltage of 3000 volts was then impressed between the cathode and anode and sputtering initiated at a rate of 260 angstroms per minute. Sputtering was continued for a time period of approximately 10 minutes, thereby resulting in a film 2600 angstroms in thickness. Following the sputtering treatment, the specific resistivity in ohm-centimeters was measured and found to be approximately 100. Next, the sputtered film was annealed for 3 hours at a temperature of approximately 600 C. and the resistivity again measured. Subsequent to annealing the resistivity of the material was found to be approximately 10 -10 ohm centimeters.

I claim:

I. A technique for the decomposition of stoichiometric gallium phosphide films comprising the steps of depositing polycrystalline gallium phosphide at a rate within the range of to 380 angstroms per minute upon a substrate member heated to a temperature within the range of 25 to 650 C., the lower rates corresponding with the higher temperatures and the converse, substantially as shown in FIG. 3, by sputtering from a target of gallium phosphide and annealing the resultant deposited film at a temperature within the range of 600 to 700 C.

2. Technique in accordance with claim 1 wherein the thickness of said deposited film ranges from 2500 to 80,000 angstroms.

3. Technique in accordance with claim 1 wherein said gallium phosphide film is doped with an impurity.

4. Technique is accordance with claim 1 wherein said gallium phosphide is deposited at a rate of approximately 260 angstroms per minute.

5. Technique in accordance with claim 4 wherein said deposited film is annealed at a temperature of 600 C.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION q 6o7 699 Dated Inventofls) Jacob Sosniak It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Patent No.

In column 4, line &3, Claim 1 reads:

1, A technique for the decomposition of should read l. A technique for the deposition of Signed and sealed this 11 th day of April 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents USCOMM-DC 60375-969 ORM PO-105O (10-69) v us. sovzmmrm PRINTING ornczloss o-ase-au 

2. Technique in accordance with claim 1 wherein the thickness of said deposited film ranges from 2500 to 80,000 angstroms.
 3. Technique in accordance with claim 1 wherein said gallium phosphide film is doped with an impurity.
 4. Technique is accordance with claim 1 wherein said gallium phosphide is deposited at a rate of approximately 260 angstroms per minute.
 5. Technique in accordance with claim 4 wherein said deposited film is annealed at a temperature of 600* C. 